API degree

A hydrometer scale used to measure the density of petroleum, established by
the American Petroleum Institute. Symbol, °API.

When the U.S. Bureau of Standards standardized the Baumé
hydrometer scale, it was “discovered that most of the hydrometers in use in the American
petroleum industry had been erroneously manufactured to a modulus of 141.5 rather than 140. By 1921 this condition had become so firmly entrenched that the only seeming remedy was to recognize the scale in predominate use and rename it.”¹

where “G” stands for the specific gravity of the liquid at 60°F in
relation to water at 60°F.

1.
Ernest L. Ruh, James J. Moran and Robert D. Thompson.
Measurement problems in the instrument and laboratory apparatus fields.
in Systems of Units. National and International Aspects.
Carl F. Kayan, editor.
Publication No. 57 of the AAAS.
Washington, D. C.: American Association for the Advancement of Science, 1959.
Page 29.